The present invention is directed generally to data processors, radio transceivers, and other circuits requiring a stable clock reference and, more specifically, to a voltage controlled oscillator (VCO) capable of linear operation at very low frequencies.
A voltage controlled oscillator (VCO) is a common circuit used to generate, among other things, clock signals for use in data processors, radio transceivers, and similar circuits. Typically, the VCO is part of a phase-locked loop (PLL) that uses feedback to provide an accurate and stable clock reference signal for other circuits. The frequency of oscillations generated by a VCO are controlled by an externally applied control voltage. Two important parameters in VCO design are linearity and sweep range. Linearity correlates the change in frequency or the VCO output to the change in the control voltage. The sweep range is the range of possible frequencies produced by VCO control voltage. A problem for many voltage controlled oscillators is that there is a trade-off between the sweep range and linearity at low frequency. For any appreciably large sweep range, a conventional VCO typically exhibits non-linearity at low frequency.
Therefore, there is a need in the art for an improved voltage controlled oscillator that has improved linearity at low frequency.
More particularly, there is a need in the art for a voltage controlled oscillator that has a large sweep range while maintaining high linearity at low frequency.
To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide a voltage controlled oscillator (VCO) capable of receiving a +V(IN) control voltage and a xe2x88x92V(IN) control voltage and outputting a VCO output signal having a frequency of oscillation determined by the +V(IN) control voltage and the xe2x88x92V(IN) control voltage. According to an advantageous embodiment of the present invention, the VCO comprising: 1) a storage capacitor capable of being charged linearly by a constant charge current and discharged linearly by a constant discharge current; 2) a comparator capable of comparing a voltage on the storage capacitor to an upper threshold voltage and a lower threshold voltage, wherein an output of the comparator drops to a negative saturation voltage (xe2x88x92V(SAT)) when the storage capacitor voltage rises above the upper threshold voltage and the comparator output rises to a positive saturation voltage (+V(SAT)) when the storage capacitor voltage drops below the lower threshold voltage; 3) a constant charge current source capable of injecting the constant charge current to the storage capacitor when the comparator output rises to the positive saturation voltage; and 4) a constant discharge current source capable of draining the constant discharge current from the storage capacitor when the comparator output drops to the negative saturation voltage.
According to one embodiment of the present invention, the comparator output is coupled to the VCO output.
According to another embodiment of the present invention, the constant charge current is determined by the +V(IN) control voltage.
According to still another embodiment of the present invention, the constant discharge current is determined by the xe2x88x92V(IN) control voltage.
According to yet another embodiment of the present invention, the constant charge current source comprises a PNP-type bipolar junction transistor having a base coupled to the +V(IN) control voltage, an emitter coupled to the comparator output via a load resistor, and a collector coupled to the storage capacitor.
According to a further embodiment of the present invention, the constant charge current source comprises a NPN-type bipolar junction transistor having a base coupled to the xe2x88x92V(IN) control voltage, an emitter coupled to the comparator output via the load resistor, and a collector coupled to the storage capacitor.
According to a still further embodiment of the present invention, the comparator comprises: 1) an operational amplifier having an inverting input coupled to the storage capacitor; 2) a first resistor (R1) having a first terminal coupled to ground and a second terminal coupled to a non-inverting input of the operational amplifier; and 3) a second resistor (R2) having a first terminal coupled to an output of the operational amplifier and a second terminal coupled to the non-inverting input of the operational amplifier, wherein the operational amplifier output comprises the comparator output.
According to a yet further embodiment of the present invention, the upper threshold voltage is determined by the product: [(R1)/(R1+R2)][+V(SAT)].
In one embodiment of the present invention, the lower threshold voltage is determined by the product: [(R1)/(R1+R2)][xe2x88x92V(SAT)].
In another embodiment of the present invention, the constant charge current is determined by a difference between the positive saturation voltage and the +V(IN) control voltage and constant discharge current is determined by a difference between the negative saturation voltage and the xe2x88x92V(IN) control voltage.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.
Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms xe2x80x9cincludexe2x80x9d and xe2x80x9ccomprise,xe2x80x9d as well as derivatives thereof, mean inclusion without limitation; the term xe2x80x9cor,xe2x80x9d is inclusive, meaning and/or; the phrases xe2x80x9cassociated withxe2x80x9d and xe2x80x9cassociated therewith,xe2x80x9d as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term xe2x80x9ccontrollerxe2x80x9d means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases.